theorem Th40:
  {}/b = {} & a/1 = a
proof
A1: b <> {} implies RED(b,{}) = 1 by Th26;
  RED({},b) = {} by Th26;
  hence {}/b = {} by A1,Def10;
  RED(1,a) = 1 & RED(a,1) = a by Th24;
  hence thesis by Def10;
end;
